The present invention relates to the field of electrical test probes. More particularly, the present invention relates to an active differential test probe with a transmission line input structure.
An electrical signal will change when a test instrument, such as an oscilloscope, is connected to the circuit that generates the signal. For example, if a bare wire is connected between a circuit and an oscilloscope, the wire and the input circuitry of the oscilloscope effectively add a load resistance and a shunt capacitance to the circuit. This reduces the measured voltage and affects measurements of dynamic timing characteristics, such as pulse rise time. For this reason, a test probe that minimizes the loading effects on the circuit is generally used when a test instrument is connected to a circuit. Several general types of test probes have been developed.
With a high-impedance test probe, it is possible to take a small sample of the signal without appreciably loading the circuit being measured. A high-impedance test probe consists of a large value resistor and an input capacitor coupled, in parallel, to a test point in the circuit. A high-impedance test probe, however, is not suited for high-frequency measurements because of the relatively high value of its input capacitance.
A low-impedance test probe is better suited for measurement of high frequency signals. A low-impedance test probe consists of a low-value input resistor in series with the signal conductor of a low-loss coaxial cable that is treated as a terminated transmission line. One limitation of the low-impedance test probe is that it may be used only at a test point with a relatively low source resistance. Another disadvantage is that the low-impedance test probe is a single ended test probe. An additional limitation of the low-impedance test probe is that the frequency is limited to the resonant frequency of the probe input capacitance in series with the ground lead inductance.
An active test probe represents another approach for obtaining accurate measurements of high frequency signals. An active test probe includes a resistive/capacitive divider network coupled between a test point and an amplifier with a high input impedance. One limitation of the active test probe, however, is that it is not possible to design an amplifier with the required high input impedance at very high frequencies. Another limitation of the active test probe is that high frequency signals can be distorted because of electromagnetic wave reflection. This signal distortion results from the fact that as frequency increases, the input structure becomes large with respect to the electrical wavelength.
The active test probe design has additional limitations when it is used in differential test probes. A differential test probe measures two signals and outputs a third signal representing the difference between the first signal and the second signal. An active differential test probe consists of two resistive/capacitive divider networks, one for each signal to be measured, and a differential amplifier. To function properly the two divider networks of the differential test probe must be accurately matched. In practice, however, the difficulty of properly matching the two divider networks can be a significant limitation. Another limitation is that high frequency signal distortion from electromagnetic wave reflection can be a significant problem when sampling two spaced-apart test points. In this situation, it may be physically impossible to keep the input structures small with respect to the electrical wavelength.
As mentioned above, a differential probe measures the difference between two input signals. For this purpose, two probe tips are needed. Most prior art dual tip systems are plug-in devices. In order to adjust the distance between the two tips, the tips are able to slide or swivel. The problem with these systems is that the tips often slide or swivel by themselves when the user does not want them to move.
Accordingly, there is a need for an active differential test probe with a transmission line input structure that does not require a high impedance amplifier, matched input networks, and small input structures.